ABSTRACT Mechanical ventilation (MV) is a life-saving therapy for respiratory failure but also leads to severe physiological and morphological alterations in the lung, known as ventilator-induced lung injury (VILI). Inflammation is a key component of acute lung injury (ALI), and work from our group and others has shown that MV leads to hypoxemia, cytokine production, neutrophil recruitment, and lung injury, even at low tidal volumes. Although neutrophils are most often associated with the pathology of VILI, our proposed study aims to demonstrate a regulatory, tissue-reparative function of neutrophils in the context of VILI and susceptibility to infection. Indeed, the immunomodulatory effects of MV with regards to infection remain unknown. Preliminary data from our lab show that MV stimulates rapid but transient neutrophil recruitment, activation and deposition of extracellular DNA in the airspaces, termed neutrophil extracellular traps (NETs), and we found that these NETs provide protection against infection with Pseudomonas aeruginosa. In particular, this protective effect of neutrophil activity lasts several days after MV, at a time when neutrophils can no longer be detected in the airspaces. Thus, we propose the novel hypothesis that MV-induced neutrophil activation and NET deposition in the airways constitute an innate immune response to tissue damage that counterbalances post-trauma susceptibility to infection. In Specific Aim 1, we will demonstrate that neutrophils are the source of MV-induced NETs using depletion, adoptive transfer and PCR- based methods to detect NET-DNA, as well as quantify the half-life of NETs in the airspaces. Despite the known antibacterial properties of NETs and their induction in response to MV, no study has investigated the impact of NETs on subsequent infection. Thus, in Specific Aim 2, we will experimentally ablate NETs and NET components, both in vivo and in vitro, to determine their impact on bacterial infection of the lung. Thus, using MV as a model of sterile ALI with transient neutrophil extravasation into the air spaces, our work aims to identify an as-yet-unidentified protective effect of NET production and thereby improve the design of future clinical treatments to increase lung function and prevent infection.